In the evolved universal terrestrial radio access network (E-UTRAN, also known as long term evolution LTE) there are ongoing discussions for a new carrier type for Release 11 or 12 or LTE-Advanced (LTE-A). Future deployments of LTE-A are to include heterogeneous deployments in which a macro eNB (MeNB) uses higher transmission power and so covers a larger geographic area in which lie one or more small cells, which may alternatively be termed pico eNBs, micro eNBs, home eNBs and the like. Any of these may be implemented as a remote radio head (RRH) of the MeNB. These small cells transmit with relatively low power and so can serve only a relatively smaller geographic area, which LTE-A terms an enhanced local area (eLA). Any given eLA may be served by one or more cells which may be clustered or grouped together from the perspective of the MeNB. FIG. 1 illustrates such a HetNet deployment with three non-clustered eLA eNBs 12, each under assistance of one MeNB 14. There is also shown one mobile terminal, more generally termed a user equipment (UE) 10, which is in close proximity to one of the small cells.
With respect to frequency, the MeNB 14 may be considered as occupying a primary component carrier (PCell) and each small cell 12 occupies a secondary component carrier (SCell) that is frequency-distinct from the PCell. The MeNB 14 may also operate one or more SCells but not the same SCells/frequencies as those used by the small cells 12, except if the MeNB 14 also operates a SCell as a small cell in which interference with a like-frequency eLA eNB-operated SCell is mitigated by the reduced power on that MeNB-operated SCell. The SCell of the small cells may conform to the new carrier type which is not yet fully defined. As location-based services become more important to the users of the UEs, the small cells are to be used to aid in the UE obtaining its own location information which the MeNB 14 collects.
A problem arises in the FIG. 1 deployment in that the UE 10 may or may not be attached to the small cell 12, and the MeNB 14 does not know enough about the UE's location to know that it is close to a given small cell 12. To this end there are ongoing discussions concerning the small cell transmitting discovery signals which UEs can use to discover small cells for measurements that may be used for obtaining the UE location. In current LTE standards discovery signals transmitted from the eNB are used for cell identification and carry the Cell Identifier (ID). For small cell discovery signals see for example document R1-120398 by NTT DOCOMO entitled ENHANCED CELL IDENTIFICATION FOR ADDITIONAL CARRIER TYPE (3GPP TSG RAN WG1 Meeting #68; Dresden, Germany; 6-10 Feb. 2012). To preserve UE battery power there is a long periodicity on the order of a few seconds between the transmitted discovery signals. This is because when the small cell SCell is not synchronized with the MeNB PCell, meaning the UE needs to do fairly extensive and power-intensive correlation operations to detect the discovery signal.
What is needed is a more efficient way for the MeNB to know when a UE is near or under the coverage of an eLA eNB so as not so consume so much power at the UEs which detect the small cells.